1. Field of The Invention
The present invention relates to an improved process for forming a large area amorphous silicon deposited film excelling in semiconductor characteristics which includes the step of intermittently irradiating inert gas plasma during film formation.
2. Related Background Art
As for the so-called amorphous silicon (a-Si in other words), there are various advantages, which cannot be attained in the case of a single crystal silicon, such that a film composed of a-Si (hereinafter referred to as a-Si film) can be formed not only on a glass substrate but also on other commercially available substrates at low substrate temperature; the a-Si film can be easily formed at large area; the a-Si film is superior to a film composed of the single crystal silicon (hereinafter referred to as single crystal Si film) with respect to light absorption; and the property of the a-Si film is isotropic but does not exhibit polarity. In addition, the a-Si film is free of grain boundary which is present in a polycystal silicon film has. Further as for the a-Si film, there is an advantage that it can be relatively easily produced at a reduced production cost.
In view of these advantages, there are a number of proposals to use an a-Si film as a constituent element in solar cells, in scanning circuits of image-reading devices such as line photosensors, area photosensors, etc., in TFTS, TFT arrays or matrices used not only in operation circuits of liquid crystal displays but also in switching circuits of photosensors, and electrophographic photosensitive members.
As the method of forming an amorphous silicon deposited film usable in such semiconductor devices, there are known or reported plasma CVD methods such as RF plasma CVD method (so-called glow discharge decomposition method), microwave plasma CVD method, etc., reactive sputtering method, light-induced CVD method, thermal-induced CVD method, vacuum evaporation method and electron cyclotron resonance CVD method.
In order to form an a-Si film to be used as a constituent semiconductor layer of an amorphous silicon semiconductor device by means of the plasma CVD technique using silane gas such as SiH.sub.4, Si.sub.2 H.sub.6, etc. as the film-forming raw material gas, it is generally recognized that the RF plasma CVD method and the microwave plasma CVD method are appropriate. The reactive sputtering method in which a Si-target is sputtered within Ar plasma in the presence of hydrogen gas is also recognized as being appropriate in order to form such a-Si film.
Other methods, that is, the light-induced CVD method, thermal-induced CVD method, vacuum evaporation method and electron cyclotron resonance CVD method, are practiced only in experimental scale but not employed in industrial scale.
Now, the a-Si films to be used in semiconductor devices which are formed according to these film-forming methods are mostly hydrogenated a-Si films containing 10 atomic % or above of hydrogen atoms. In other words, it is considered that a-Si films usable as the electronic constituent materials exhibiting the characteristics required for obtaining a-Si semiconductor devices are mostly those containing 10 atomic % or above of hydrogen atoms.
In order to form such a-Si films, the foregoing plasma CVD technique is widely used in various sectors since there are various advantages that the constitution of the apparatus used is relatively simple and can easily designed and the film-forming conditions can be relatively easily controlled. In the case of forming such a-Si film by means of the plasma CVD technique, the film formation is carried out, for example, in the following manner. That is, SiH.sub.4 gas or Si.sub.2 H.sub.6 gas (if necessary, diluted with hydrogen gas) as the film-forming raw material gas is introduced into a deposition chamber containing a substrate on which a film is to be formed, wherein energy of a high frequency of 13.56 MHz or 2.45 GHz is applied to cause plasma by which the film-forming raw material gas is decomposed to produce active species, thereby causing the formation of an a-Si deposited film on the substrate. In this case, the resulting a-Si film can be easily made to be of n-type or p-type by mixing an proper doping gas such as PH.sub.3, B.sub.2 H.sub.6, BF.sub.3, etc. with the film-forming raw material gas.
In any case of forming an a-Si semiconductor film, it is known that especially the substrate temperature, among other film-forming parameters, greatly influences the quality of the resulting a-Si semiconductor film and it is extremely important to maintain the entire of a substrate uniformly at a predetermined temperature particularly in the case of forming a desirable a-Si semiconductor film of large area. However, it is very difficult to maintain all of a large area substrate uniformly at a predetermined temperature in order to form such large area a-Si semiconductor film especially by means of a plasma CVD method. That is, in the case of forming a desirable large area a-Si semiconductor film on a large area substrate, for example, of 100 mm to 1000 mm in size by means of the plasma CVD method, wherein the film formation is usually performed at a inner pressure of 0.1 to 1 Torr and at a predetermined substrate temperature in the range of 200.degree. to 300.degree. C., it is difficult to maintain the entire substrate of such large area size uniformly at said temperature under the condition of such low inner pressure and it takes a long period of time until the predetermined uniform substrate temperature is attained for the entire of the large area substrate. And in the case of repeatedly forming a large area a-Si semiconductor film while replacing the previous large area substrate by a new large area substrate in each film forming operation, it is difficult to maintain all of the new large area substrate uniformly at the identical substrate temperature in each case. In order to repeatedly form a desirable large area a-Si semiconductor film, other film-forming parameters than the above substrate temperature are necessary to be properly controlled so that uniform plasma is produced along the entire surface of a large area substrate on which the large area a-Si semiconductor film is to be formed in each film-forming operation. However, it is difficult to control those parameters uniformly in the respective film-forming operations. Thus, it is extremely difficult to mass-produce a desirable large area a-Si semiconductor film having a uniform film property at a high yield. Particularly, each of the resulting large area a-Si semiconductor films is liable to be such that is varied with respect to the film property all over the large area substrate, and the resulting large area a-Si semiconductor films become such that are varied with respect to the film property. This situation is problematic, for example, in the case of forming a multi-layered semiconductor device comprising a plurality of a-Si semiconductor films being stacked on a large area substrate. That is, it is difficult to mass-produce a desirable multi-layered semiconductor device each of which constituent a-Si semiconductor films having a uniform film property all over the large area substrate, which exhibits uniform characteristics and which is free of local occurrence of light deterioration (which is the so-called Stabler-Wronski effect) at a high yield, since to maintain all of the large area substrate uniformly at a predetermined temperature, to secure uniform distribution of plasma along the entire surface of the large area substrate and to secure uniformity of the film-forming conditions upon forming each of the constituent a-Si semiconductor films are difficult as above described.
Particularly, the local occurrence of light deterioration at the constituent a-Si semiconductor film is problematic even in the case where the above multi-layered semiconductor device is an image-reading device in which light having a relatively weak intensity is irradiated, but it is serious in the case where the above multi-layered semiconductor device is a solar cell in which sun light (which has a strong intensity) is irradiated. In the case where the above multi-layered semiconductor device is an electrophotographic photosensitive member, the problem relative to unevenness in the characteristics of the constituent a-Si semiconductor film leads to reproducing undesirable images which are not even in density or which are easily deteriorated. In the case where the above multi-layered semiconductor device is a thin film transistor (TFT), the problem relative to unevenness in the characteristics of the constituent a-Si semiconductor film makes the TFT such that does not exhibit uniform TFT characteristics. Similarly, in the case where a plurality of the above multi-layered semiconductor devices are arranged in matrix-like state to be a device used in the operation circuit of a liquid crystal display, the problem relative to unevenness in the characteristics of the constituent a-Si semiconductor film results in making said device such that it exhibits varied characteristics.
In order to solve the above problems caused by the a-Si semiconductor film formed by the conventional plasma CVD method, there is a proposal to improve its film property by subjecting the resultant a-Si semiconductor to after-treatment. For instance, the preliminary report for the 1988 Autumn Conference of the Applied Physics Association 5p-2f-1 and the preliminary report for the 1990 Spring Conference of the Applied Physics Association 31a-2D-8 & 31a-2D-11 report methods in which the previously formed a-Si film is subjected to repetition of H.sub.2 plasma treatment. According to these methods, the a-Si film is crystallized as the H.sub.2 plasma treatment is repeated, wherein the resulting Si film is not amorphous but crystalline. The Si crystalline film is advantageous depending upon application use, but the extent of its applicability is narrower than that of an a-Si film. As for the crystalline Si film, there are disadvantages such that it is inferior to the a-Si film with respect to light absorption; it is poor in uniformity because of having grain boundaries (the a-Si film is free of such problem); and it is not desirable to be used in light receiving devices. In addition to these disadvantages, there is also a disadvantage that uncontrollable crystallization is caused in the case of repeatedly treating the a-Si film with H.sub.2 plasma.
Thus, the proposed methods are not effective in order to solve the foregoing problems in the prior art.